The invention relates to a method and a system as set forth in the preambles of the appended independent claims for detecting one or more gases or gas mixtures and/or for measuring the concentration of one or more gases or gas mixtures.
Typically, the detection of various gases and gas mixtures, as well as concentrations thereof, is performed by using detectors based on photoacoustic phenomenon. When light falls in a gas-filled chamber, which contains a gas to be analysed at a partial pressure px and a carrier gas at a partial pressure pN (typically often nitrogen), radiation is absorbed by the gas px. The purpose is to measure the partial pressure px of a gas x. After the absorption process, energy converts to a thermal motion at a certain time constant τ (e.g. 10−5 s). Thus, the entire gas has its temperature rising by ΔT per unit time. The temperature rise also brings about a pressure rise Δp, which can be converted to a photoacoustic signal or which can be used as a photoacoustic signal. The term light used in this application should be perceived in a general sense, as in reference to electromagnetic radiation, comprising e.g. ultraviolet radiation, visible light, infrared radiation, and microwaves. Respectively, the term light beam covers electromagnetic beams in general, and the term light source encompasses generally various sources of electromagnetic radiation.
A typical photoacoustic detector comprises a chamber, which can be supplied with a gas or gas mixture to be analysed, a window for admitting modulated or pulsed infrared radiation or light into the chamber, and a pressure sensor arranged to measure pressure variations in the chamber generated by absorbed infrared radiation or light. The pressure sensor is typically a microphone, in which the movement of a thin Mylar or metal diaphragm is measured, for example capacitively. A photoacoustic detector can be used for measuring or detecting infrared radiation generally, but one specific and important application of such a detector involves measuring or detecting gases or gas mixtures, for example in relation to the quality or pollutions of air.
Typically, a photoacoustic detector is connected to a spectrometer, whereby the spectrometer can be used for detecting various gases in a gas mixture and/or for measuring the concentrations or partial pressures of gases. If a photoacoustic detector is to be used for measurements without a spectrometer, the detector must be selective for a gas to be examined. According to prior known techniques, the selectivity in a photoacoustic process is typically achieved by using a laser as its radiation source, which has its wavelength coinciding with the wavelength of any absorption line of the gas. Another prior known option is to employ an optical filter in connection with a broadband emitter, whereby a desired wavelength range is established by selecting an optical filter having desired filtering properties.
A problem in laser-based measurement is typically its laborious use. Another problem with lasers is the limited range of available wavelengths, thereby limiting the range of substances that can be analysed.
A problem with an optical filter is typically poor quality filtration, the optical filter not being optimal in terms of its selectivity. FIG. 1 illustrates the operation of an optical filter. If there are several gas absorptions within a filter area, the signals thereof are summed and the result will be a sum of the concentrations of the gases. A photoacoustic signal represents the area of a spectrum transmitted through the filter. The signal is weak because a signal is generated only by a fraction of the spectrum of a substance being analysed.
Kovalyov and Klebleyev (A. A. Kovalyov, N. R. Klebleyev, Resonant optoacoustic detector in nondispersive gas analyzer scheme, Infrared Physics & Technology 38, 1997, pp. 415-421) have proposed the replacement of an optical filter with a chamber, containing a gas to be measured at a known pressure p0x. According to what is set forth in this publication, the measuring chamber of a photoacoustic detector is supplied with a sample of gas to be analysed, the objective being to measure the analysable gas mixture contained therein for a partial pressure px of its gas x. The chamber is supplied with a pulsed light beam, the first of which is passed, prior to the measuring chamber, through a void reference space and the second of which is passed, prior to the measuring chamber, through a filter chamber containing the gas x to be analysed at a known pressure p0x. The photoacoustic detector, and especially its measuring chamber, has been arranged in such a configuration that, if the gas sample supplied into the measuring chamber does not contain a gas identical to the gas presently in the filter chamber, the chamber shall not develop a pressure variation detected by a microphone in the measuring chamber. If the gas sample arranged into the measuring chamber does contain a gas identical to that in the filter chamber, a photoacoustic pressure variation, i.e. a photoacoustic signal, will be detected by the microphone.
The most notable problem in the solution proposed by Kovalyov and Klebleyev is its insensitivity, which is at best equal to that of a conventional method comprising only a photoacoustic measuring chamber. The reason for this is that, unlike an optical filter, a filter chamber is not capable of providing a 100% absorption. If the absorption of a filter chamber is increased by means of pressure to the proximity of 100%, the breadths of absorption lines shall increase, thus impairing the method as regards its selectivity. In addition, the pressure sensor employed is a capacitive microphone, wherein sensitivity is limited by a stiffness of the diaphragm and a capacitive measurement of the diaphragm motion. In a capacitive microphone, some of the energy of diaphragm motion is spent for the alternating flow of gas in and out between diaphragm and electrode.
Publication U.S. Pat. No. 4,373,137 also discloses a method for measuring a photoacoustic pressure signal generated in a two-section measuring chamber by a pulsed light beam passing through a sample chamber. The pressure difference between successive measuring chambers is measured capacitively by means of a diaphragm, which results in a poor sensitivity of the method as described above.
In addition, patent publications U.S. Pat. No. 5,055,688 and U.S. Pat. No. 4,682,031 describe measuring systems, including a measuring device which comprises sample and reference chambers set side by side for conducting modulated light beams therethrough into a measuring chamber. A problem with the solutions disclosed in these publications is also that the pressure sensors used therein are not sufficiently sensitive, being based on a capacitive measurement of the diaphragm motion.
Patent publication U.S. Pat. No. 6,452,182 further discloses a photometer, comprising a plurality of successively arranged measuring chambers, containing an isotope of the gas to be examined which is different from the one in the preceding chambers. The described solution is typically aimed at enabling a measurement of various isotopes of a gas with a single device in a single measurement. However, a problem with the measuring systems disclosed in the publication is the insensitivity thereof, for the reason mentioned above.
Another problem in prior known photoacoustic detectors is the sensitivity thereof to external sounds. Therefore, the gas space in prior known photoacoustic detectors must be sealed, i.e. it is not possible to use an open gas space but, instead, the gas space functioning as a sample space must be separately filled with a sample to be measured, typically gas.